A touch panel is used in many cases as an input device of a computer system. The touch panel is mounted on a display to detect a coordinate position on the display and acquire a detection signal corresponding to the detected coordinate position. The touch panel permits a simple and intuitive direct input of a coordinate position.
There are various types of touch panels, such as a resistance-film type, an optical type, a capacity-coupling type, etc. Among those types, a resistance-film touch panel is used in many cases because the resistance-film touch panel has a simple structure and a simple control system.
The resistance-film touch panel includes a low resistance type, which is classified into a four-wire system, a five-wire system, an eight-wire system, etc. Unlike a resistance-film touch panel using the four-wire system or the eight-wire system, a resistance-film touch panel using the five-wire system (hereinafter, referred to as “five-wire resistance-film touch panel) has a conductive film of an upper board arranged on an operation face side, which conductive film is exclusively used for reading a potential. Accordingly, five-wire resistance-film touch panel does not have a problem of an edge slide, which is a disadvantage in other systems such as the four-wire system and the eight-wire system. Thus, the five-wire resistance-film touch panel can be used in an application, which requires use in a severe environment and a long service life.
FIG. 1 illustrates a structure of the five-wire resistance-film touch panel. The five-wire resistance-film touch panel 1 includes an upper board 11 and a lower board 12. A transparent resistance-film 22 is formed on an entire surface of a glass substrate 21 of the lower board 12. X-coordinate detection electrodes 23 and 24 and Y-coordinate detection electrodes 25 and 26 are formed on the transparent resistance-film 22. On the other hand, a transparent resistance-film 32 is formed on a film substrate 31 of the upper board 11, and a coordinate detection electrode 33 is formed on the transparent resistance-film 32.
First, a voltage is applied to the X-coordinate detection electrodes 23 and 24, which results in a potential distribution generated in an X-coordinate direction in the transparent resistance-film 22 of the lower board 12. Accordingly, the X-coordinate of a position where a portion of the upper board 11 is in contact with the lower board 12 can be detected by detecting a potential on the transparent resistance-film 22 of the lower board 12. Then, a voltage is applied to the Y-coordinate detection electrodes 25 and 26, which results in a potential distribution generated in a Y-coordinate direction in the transparent resistance-film 22 of the lower board 12. Accordingly, the Y-coordinate of the position where the portion of the upper board 11 is in contact with the lower board 12 can be detected by detecting a potential on the transparent resistance-film 22 of the lower board 12.
It is an issue for the above mentioned touch panel as to how to generate a uniform potential distribution in the transparent resistance-film 22 of the lower board 12. In order to make uniform the potential distribution in the transparent resistance-film 22 of the lower board 12, there is suggested a plurality of potential distribution correction patterns being arranged on a periphery of the lower board 12 (for example, refer to Patent Document 1). Additionally, there is suggested a common electrode being arranged to surround an input surface (for example, refer to Patent Document 2). Further, there is suggested an opening part being formed in an insulation film provided on a transparent resistance-film to apply a potential through a portion where the opening parts is formed (for example, refer to Patent Document 3).
Patent Document 1: Japanese Laid-Open Patent Application No. 10-83251
Patent Document 2: Japanese Laid-Open Patent Application No. 2001-125724
Patent Document 3: Japanese Laid-Open Patent Application No. 2007-25904
A coordinate input device is required to be smaller in an outer frame configuration due to miniaturization of an apparatus into which a coordinate input device is incorporated. However, it is difficult to reduce an outer frame size of a conventional coordinate input device because a plurality of potential distribution patterns must be arranged on a periphery of the coordinate input device.
The method of providing a common electrode, which is arranged to surround an input surface, has a problem in that a potential distribution of a transparent resistance-film becomes uneven if a resistance ratio of a resistance of the transparent resistance-film and a resistance of a pattern resistor is not sufficiently large.
The method of providing an opening part in an insulation film may solve the above-mentioned problems, but a manufacturing process becomes complex and, especially, variation in resistance values due to variation in materials and manufacturing process may deteriorate a product performance, which reduces a yield rate.